What is Septic Arthritis
Septic arthritis is an acute bacterial infection of a joint space that constitutes an orthopedic emergency, requiring immediate diagnosis and treatment because bacterial proliferation and metabolites can cause irreversible cartilage destruction within hours to days. 1, 2
Definition and Pathophysiology
Septic arthritis refers to infection of one or more joints, most commonly caused by bacteria, though viral and fungal etiologies can occur. 3, 4 The condition arises predominantly through hematogenous spread of bacteria to the joint space. 1 Bacterial enzymes and inflammatory mediators rapidly degrade articular cartilage, leading to permanent joint damage if treatment is delayed. 2, 5
Epidemiology and Mortality
- The incidence ranges from 2 to 20 per 100,000 people annually, with higher rates in pediatric populations. 1, 5
- Mortality rates are substantial: 7% in patients ≤79 years and 22-69% in those >79 years for knee septic arthritis alone. 3
- Overall mortality across all joints ranges from 2-15%. 4
- Poor functional outcomes (amputation, arthrodesis, prosthetic surgery, severe functional deterioration) occur in 24-33% of patients. 3
Common Causative Organisms
Staphylococcus aureus is the most common pathogen across all age groups, accounting for approximately 53% of cases (36% MSSA, 17.6% MRSA). 6, 7 Other important pathogens include:
- Group B Streptococcus (7.3% of cases, particularly in shoulder infections) 7
- Coagulase-negative Staphylococci (13% of cases) 7
- Kingella kingae in children <4 years of age 1, 2
- Group B Streptococcus in neonates 1, 2
- Salmonella species in patients with sickle cell disease 1, 8
- Streptococci and Gram-negative bacilli to a lesser extent 4
Clinical Presentation
The classic triad of fever, pain, and diminished mobility occurs in only approximately 50% of cases, making diagnosis challenging. 1, 2 The typical presentation is acute monoarticular joint pain, swelling, and fever. 3, 6, 5
Most Commonly Affected Joints
- Knee joint (most frequent) 1, 7
- Hip joint (second most common, particularly critical due to risk of femoral head necrosis) 1, 2
- Shoulder joint (third most common, with MRSA being the leading causative organism specifically in this joint) 7
Risk Factors
Key risk factors that increase susceptibility include:
- Age >80 years (associated with dramatically higher mortality) 3, 6
- Diabetes mellitus 3, 6
- Rheumatoid arthritis 3, 6
- Recent joint surgery or prosthetic joint 3, 6
- Skin infection 3, 6
- Immunosuppressive medication use 6
- HIV infection 3
- Osteoarthritis 3
- Recent intra-articular injection 3
- Smoking 3
Concomitant Infections
Concomitant joint and bone infections are extremely common in children, occurring in >50% of pediatric cases. 1, 2 This includes:
- Concurrent osteomyelitis (present in up to 58% of pediatric elbow septic arthritis cases) 2
- Subperiosteal abscess formation 1
- Soft tissue involvement and pyomyositis 2
Pediatric Considerations
In children ≤18 months of age, transphyseal vessels allow infection to spread from metaphysis to epiphysis and subsequently to the joint space. 1 In neonates and infants, septic arthritis frequently occurs secondary to spread of osteomyelitis into the adjacent joint. 1
Critical Clinical Pitfall
A delay in diagnosis and treatment results in permanent morbidity and mortality. 6 The urgency stems from the fact that bacterial proliferation can cause irreversible cartilage damage within hours to days, making septic arthritis a true musculoskeletal emergency requiring immediate intervention. 2, 8, 5